September 7, 2012

Deep-see Canyons Are Reshaped by Fish Trawling

Trawling is changing the topography and the environment of deep-sea canyons on the continental slope, a new study from the University of Barcelona finds.

The study, published in Nature, contends that marine mountainsides are looking more like plowed fields, which changes the habitats of deep-sea creatures. The process rivals landslides and storms for re-shaping the slopes.

Fishing fleets have been trawling the Mediterranean along the coast of Spain for years, dragging nets along the flat, shallow coastal sea floor. In the 1960's, they started to pursue shrimp farther offshore and into rugged canyons up to 800 meters deep. Until now, the impact this trawling had on the rough canyon terrain was a mystery.

Geoscientists were surveying off the coast of Spain in 2006 when they found smooth slopes, which they attributed to an underwater cascade. The only problem was that one slope was smoothed in the lee of the supposed cascade.

Pere Puig, a marine geologist at the Institute of Marine Sciences in Barcelona, realized that the anomalies occurred in a trawling zone and theorized that the trawlers were scraping silt off the ridge tops and dropping it into the canyons. This study is the first to address the changes to undersea terrain created by trawling.

The researchers measured silt flow in the canyons for six months, taking core samples from the sea floor and videotaping in the canyons. They plotted the silt disturbances on a high-resolution map of the canyons and compared this with four years of detailed fishing records and satellite navigation tracks.

What they found was higher silt flow during hours when the trawling fleet operated and smoother canyon walls in areas with the greatest trawling activity. Untrawled areas, by contrast, were dominated by a series of valleys. They also found different sediments in trawled and untrawled areas. The scientists estimate that trawling has doubled the amount of sediment flowing down into the canyons since the 1970's.

In areas where industrialized trawling has been taking place since the mid-1960s, the scientists found the practice displaced 5,400 tons of sediment in just 136 days they monitored.

“Changes in land use during the 20th century resulted in a reduction of landscape diversity almost everywhere through the spreading of arable land and timber plantations, which led to gradual disappearance of both major and minor elements of topography. Bottom trawling has been compared to forest clear-cutting, although our results suggest that a better comparison might be intensive agricultural activities. The frequent repeated trawling (plowing) over the same ground, involving displacement of sediments owing to mechanical redistribution, ultimately causes the leveling of the surface and produces morphological effects similar to those of a (plowed) farmer's field."

The problem is, while plowing happens a few times a year, trawling can occur almost daily.

Elliot Norse, chief scientist at the Marine Conservation Institute in Bellevue, Washington, asserts that smoothing out the structure of marine canyons will reduce the number of species that can live there. If shallower waters are any guide, it will also change the makeup of species.

Bottom trawling displaces or harms some marine species, causes pollutants to mix into plankton and move into the food chain, and creates harmful algae blooms or oxygen-deficient dead zones.

"Big fish like complex habitats," says Callum Roberts of the University of York, "things like prawns and scallops live fast, die young and like their habitats open and unstructured."

Researchers do not yet know what species the prawns will be replacing. The team is planning surveys of the biodiversity of trawled and untrawled slopes next.

This way of fishing may not be sustainable. In 2011 the United Nations Food and Agriculture Organization listed the shrimp as overfished and conservation groups support a total ban of deep-sea trawling, though some marine managers think that the species can recover if left alone for long enough.

In July, the European Commission said it wanted to ban the activity throughout the waters of the European Union, but Javier Garat, secretary-general of Spain's Fishing Confederation (Cepesca) says that industrial fishing groups would prefer regional bans, such as those enacted over the past decade in marine reserves, accompanied by scientific monitoring. They will lobby to modify the proposal when it is considered by the European Parliament and European Council this autumn.

Puig agrees that bans should be evaluated on a case-by-case basis rather than as a blanket approach because sustainable fisheries may emerge in some places where the geological and ecosystem damage is "already done".